1. Field of the Invention
Embodiments of the present invention relate to methods for forming semiconductor packages, and semiconductor packages formed thereby.
2. Description of the Related Art
The strong growth in demand for portable consumer electronics is driving the need for high-capacity storage devices. Non-volatile semiconductor memory devices, such as flash memory storage cards, are becoming widely used to meet the ever-growing demands on digital information storage and exchange. Their portability, versatility and rugged design, along with their high reliability and large capacity, have made such memory devices ideal for use in a wide variety of electronic devices, including for example digital cameras, digital music players, video game consoles, PDAs and cellular telephones.
While a wide variety of packaging configurations are known, flash memory storage cards may in general be fabricated as system-in-a-package (SiP) or multichip modules (MCM), where a plurality of die are mounted on a substrate. The substrate may in general include a rigid base having a conductive layer etched on one or both sides. Electrical connections are formed between the die and the conductive layer(s), and the conductive layer(s) provide an electric lead structure for integration of the die into an electronic system. Once electrical connections between the die and substrate are made, the assembly is then typically encapsulated in a mold compound to provide a protective package.
Flash memory modules may either be portable, as in the case of a land grid array (LGA) package, or dedicated, as in the case of a ball grid array (BGA) package. Portable flash memory modules are fabricated with contact fingers that allow the modules to be used as removable memory. They may be inserted into a slot in a host device, whereupon the contact fingers are brought into pressure contact with a printed circuit board in the host device to allow communication between the memory module and host device. Dedicated memory modules on the other hand are soldered, or otherwise permanently affixed to the printed circuit board of a host device.
In view of the small form factor requirements, as well as the fact that flash memory cards need to be removable and not permanently attached to a printed circuit board, such cards are often built as land grid array (LGA) packages. In an LGA package, the semiconductor die are electrically connected to exposed contact fingers formed on a lower surface of the package. External electrical connection with a host printed circuit board is accomplished by bringing the contact fingers into pressure contact with complementary electrical pads on the printed circuit board. LGA packages are ideal for flash memory cards in that they have a smaller profile and lower inductance than pin grid array (PGA) and ball grid array (BGA) packages. Further examples of typical LGA packages are disclosed in U.S. Pat. Nos. 4,684,184, 5,199,889 and 5,232,372, which patents are incorporated by reference herein in their entirety.
A side view of a conventional LGA package 40 is shown in prior art FIG. 1. One or more memory die 20 and a controller die 22 are mounted on a substrate 24 in a stacked configuration, along with one or more passive components 26. Generally, the substrate 24 may be formed of a rigid core having thin film copper layer(s) on its top and/or bottom surfaces. An electrical lead pattern may be defined in the copper layer in a desired electrical lead pattern using known photolithography and etching processes. The copper film on the bottom surface may also be used to define a plurality of contact fingers 28 for communication with a host device.
The die may be electrically connected to the substrate by wire bonds 34. Vias (not shown) are formed through the substrate to allow electrical connection of the die through the substrate to the contact fingers 28. Once the die are electrically connected, the package may be encapsulated in a mold compound 36 to form the package 40.
During the encapsulation process, the substrate and die are positioned in a mold cavity defined by top and bottom mold plates. A mold compound, for example molten epoxy resin, is then injected into the mold cavity to encapsulate the die on the substrate. A lower surface of the substrate (i.e., the surface including contact fingers 28) is positioned against the bottom mold plate, so that the lower surface of the substrate typically does not receive any mold compound. It is also known to form recesses in the top mold plate which get filled with mold compound to define a projection 38 across the top surface of the finished package 40. The projection 38 is typically used as a finger grip. Instead of a recess in the top mold plate, it is also known to provide a projection on the top mold plate which in turn forms a recess in the top surface of the finished package 40.
It may be desirable to form a projection, as in projection 38, on the lower surface of the substrate. As is known in the art, injecting mold compound both above and below the substrate requires high mold compound pressures. During the molding process, the molding machine may output an injection force typically about 0.8 tons or higher to drive the mold compound into the mold cavity above and below the substrate. Such pressures may generate undesirable mold flash on a bottom surface of the substrate, which is excess mold compound on the lower surface of the substrate where no mold compound should be. It is known to clamp down the die and substrate with a high clamping force within the mold cavity, but such high forces may cause cracks in the die and/or substrate.
Moreover, given the constant drive toward smaller form factor packages, it is presently known to employ wafer backgrind during the semiconductor fabrication process to provide very thin semiconductor die. Such die are often unable to withstand the large stresses generated during high pressure molding processes, and frequently break. It is therefore desirable to employ a lower pressure injection molding process to encapsulate the die. If it is desired to have a projection on the bottom surface of the package, it is known to encase prior art semiconductor packages within plastic lids, which may have any configuration of projections on the bottom and/or top surface(s).